1. Field of the Invention
The present invention relates to an image forming apparatus that forms images by exposing a photosensitive member.
2. Description of the Related Art
There has generally been known an image forming apparatus, such as a copying machine or a printer, which performs image formation using the electrophotographic process. In recent years, it has been demanded that an image forming apparatus of this type forms high-quality images on a plurality of types of recording sheets (transfer sheets) at a high speed with high accuracy.
An image forming apparatus using the electrophotographic process is provided with an exposure device that emits a light beam (e.g. a laser beam), and exposes a photosensitive member by the laser beam emitted from the exposure device to thereby form an electrostatic latent image on the photosensitive member. The laser beam is deflected by a rotary polygon mirror (hereinafter simply referred to as “the polygon mirror”) driven by a drive motor (hereinafter referred to as “the scanner motor”). When the laser beam is deflected by the polygon mirror, the spot formed by the laser beam is shifted on the photosensitive member in a predetermined direction. The photosensitive member is thus exposed by the deflected laser beam, whereby the electrostatic latent image is formed on the photosensitive member. The electrostatic latent image formed on the photosensitive member is visualized by toner as a toner image, and the toner image is transferred onto a recording medium. The toner image on the recording medium is heated by a fixing device whereby the toner is fixed on the recording medium.
When the toner image is heated and fixed on the recording medium, water contained in the recording medium is evaporated. When the amount of the water contained in the recording medium is reduced, the size of the recording medium is also reduced. In a case where double-sided printing is performed, an image on the front side of a recording medium is fixed, and then image formation is performed on the reverse side of the recording medium. Therefore, a difference in image size occurs between the front and reverse surfaces of the recording medium. Further, when humidity in an environment where a recording medium is placed rises, the amount of water contained in the recording medium increases, and hence the size of the recording medium also increases. For this reason, a problem occurs that an image formed in a high-humidity state and an image formed in a low-humidity state differ from each other in size with respect to the size of a recording medium.
Recently, in order to achieve high image quality, it has been demanded that an image forming apparatus is capable of aligning an image forming position on a recording medium with high accuracy (e.g. improvement of accuracy in positioning an image on a recording medium, and reduction of color shift that occurs in superimposition of a plurality of colors in a color image forming apparatus).
To meet the demand, e.g. before forming an image on the reverse side of the recording medium, image magnification is corrected according to expansion/contraction of an image formed on the front side of a recording medium such that an image size on the reverse side of the recording medium coincides with an image size on the front side of the same (see Japanese Patent Laid-Open Publication No. 2004-25841). In an image forming apparatus disclosed in Japanese Patent Laid-Open Publication No. 2004-25841, the rotational speed of a scanner motor is controlled, whereby the rotational speed of a polygon mirror is controlled. By changing the rotational speed of the polygon mirror, the magnification of the image in the sub scanning direction (i.e. the direction of rotation of a photosensitive member) is changed, whereby image shift is corrected. In double-sided printing, image magnification is adjusted (corrected), as mentioned above, whereby the image sizes of respective front-side and reverse-side output images are made equal to each other.
However, when an electrostatic latent image is formed on the photosensitive member by the method disclosed in Japanese Patent Laid-Open Publication No. 2004-25841, there arises the following problem.
FIGS. 22A and 22B are views illustrating pitch intervals in the sub scanning direction between scanning spots (exposure spots) before and after changing of the rotational speed of a polygon mirror of a conventional image forming apparatus. FIG. 22A illustrates scanning spot intervals in an apparatus in which the photosensitive member is exposed by a laser beam (single beam) emitted from a single light source provided in the exposure device. FIG. 22B illustrates scanning spot intervals in an apparatus in which the photosensitive member is exposed by a plurality of laser beams (multi-beams) emitted from a plurality of light sources provided in the exposure device.
When the rotational speed of the polygon mirror is increased in the apparatus in which the photosensitive member is exposed by a single beam (see FIG. 22A), so as to correct image magnification in the sub scanning direction, space intervals between scanning lines formed on the photosensitive member are uniformly reduced. Therefore, as the rotational speed of the polygon mirror is increased, image density is also increased. Thus, a difference in density is caused between a front-side image and a reverse-side image when forming images on the front and reverse sides of the recording medium using the same image data.
On the other hand, when the rotational speed of the polygon mirror is increased in the apparatus in which the photosensitive member is exposed by a plurality of beams (multi-beams) (see FIG. 22B), so as to correct image magnification in the sub scanning direction, a space interval between a last scanning line formed on the photosensitive member in one scan and a first scanning line formed on the photosensitive member in the next scan (the space interval will be hereinafter referred to as “the scan-to-scan line interval”) becomes different from the space interval between scanning lines formed on the photosensitive member in the single scan. In short, whenever the rotational speed of the polygon mirror is changed, the scan-to-scan interval is reduced or increased according to the rotational speed of the polygon mirror. Thus, the space intervals between scanning lines formed on the photosensitive member become non-uniform due to change in the rotational speed of the polygon mirror. More specifically, the reduced magnification makes some scanning line interval narrower, and the increased magnification makes some scanning line interval wider. This causes variation in output image density.
Thus, when the rotational speed of the polygon mirror is changed, the quality of an image formed on the recording medium is degraded.